The present invention relates to fluid circulators.
In nuclear power reactors, gas, usually, helium, is circulated from the nuclear power reactor, to take up heat from the reactor and transport it to steam generators. Established practice is to use electrically driven circulators, in which the driving motor and the support bearings are submerged in high pressure gas, which is in direct communication with the gas circuit.
The electrically driven generators have conventionally used oil lubricated gearings, one of which, of necessity, is located close to the reactor gas circuit and therefore has the potential to leak oil into this gas circuit. By careful seal design it is possible to minimize this leakage, but unfortunately it cannot be reduced to absolute zero.
A further proposal has been to use active magnetic bearings for rotor support. This would involve no lubricating fluid and therefore there will be no possibility to contaminate the reactor gas circuit. However, there are two main disadvantages which have to be weighed against this advantage. Firstly, should the magnetic bearing system be subject to a failure, for example a power failure, the machine has to be capable of coming safely to rest. It is common practice for the circulator to operate at speeds of the order of 6,000 rpm and, the requirement for safety necessitates the provision of short life auxiliary or socalled `catcher` bearings. In a high speed machine, with a large aerodynamic thrust, this provision is technically very difficult to achieve especially when the gas which is being circulated is dry helium. Secondly, the combined thrust and journal bearing necessary at at least one end of the circulator, requires a large number of power and shaft position monitoring signal leads to be physically brought out of the high pressure environment. It is difficult to achieve and maintain supply and signal integrity and the leads can become a source of unreliablity.